In order to develop a sound biotechnique for monitoring water quality that builds on the previous experiments carried out in our laboratory, a specific D. magna clone C-1 242 was used to study the effects of pollutants on phototactic behavior. In all experiments, the animals showed a stable and repeatable phototactic index approximated 0.2 in the presence and 0.4 in the absence of fish kairomones, which decreased significantly in response to pollutants. There existed no pollutant x fish kairomone interaction, indicating the changes in phototactic behavior of animals imposed by pollutants were independent of the presence of fish kairomones. The detection limits for changes in phototactic behavior of D. magna clone C-1 242 are 0.04 mg/L for copper, 0.02 mg/L for cadmium, and 0.80 mg/L for PCP, respectively, quite lower than LC50 (48 h). The changes in phototactic behavior in presence to pollutants occurred quickly(3 h) compared to the period over whole acute toxicity tests. Therefore, D. magna clone C, 242 could be potentially used to monitor water quality. Moreover, the phototactic behavior did not decrease further in the pollutant mixtures employed in our experiments compared to individual pollutants, except in the Cd-PCP treatment. This fact suggests that the formation of water quality criteria must be based upon pollutant mixture tests.